The present invention relates to a process for the production of liquid hydrocarbons from a gaseous mixture comprising carbon monoxide and hydrogen (synthesis gas), in the presence of a Fischer-Tropsch catalyst. In particular, the present invention relates to the production of liquid hydrocarbons by contacting synthesis gas with a Fischer-Tropsch catalyst in an aggregative fluidised bed.
The production of hydrocarbons by contacting synthesis gas with a Fischer-Tropsch catalyst, typically a cobalt or iron catalyst, which may be either supported or unsupported, has been known for a considerable number of years. Fischer-Tropsch processes have been operated commercially, for example, by Sasol Technology (Pty) Ltd in South Africa. Much of the early work on Fischer-Tropsch hydrocarbon synthesis was accomplished using fixed bed catalysts but in recent times attention has shifted to the use of liquid phase catalytic reactions largely because of the relative ease of removing the exothermic heat of reaction in such systems. Liquid phase Fischer-Tropsch processes typically employ a three-phase (liquid/gas/solid) slurry medium. In particular, use of fluidised beds in Fischer-Tropsch processes is known.
In a fluidised bed, solid particles are transformed into a fluid-like state by the upward passage of a gas or liquid (fluidising fluid). When the velocity of the fluidising fluid reaches a critical velocity the drag force on the solid particles equals the buoyant weight of the bed. The bed is then supported by the fluidising fluid and possesses fluid-like properties such as flowing easily and maintaining a horizontal level when the bed is tilted. In addition, low density objects may be floated on the surface of the bed. This state is known as xe2x80x9cfluidisationxe2x80x9d, and the critical superficial fluid velocity at which this occurs is termed the xe2x80x9cminimum fluidisation velocityxe2x80x9d. When the fluidising fluid is a liquid (i.e. a liquid-solid system), an increase in the rate of flow of the liquid above the minimum fluidisation conditions, generally results in a progressive expansion of the bed, which gives rise to what is termed xe2x80x9cparticulatexe2x80x9d or xe2x80x9chomogeneous fluidisationxe2x80x9d. For large diameter fluidised beds, the expansion of a particulate fluidised bed can be approximated by the relationship:
U=Utxcex5n 
where U is the superficial liquid velocity, Ut is the terminal fall velocity of the solid particles, xcex5 is the voidage of the solid particles, and n is the Richardson and Zaki exponent which is dependent upon the terminal Reynolds number, Ret and ranges from n=2.4 (Ret greater than 500) to n=4.65 (Ret less than 0.2). The exponent n is also a function of the particle-to-bed diameter ratio, d/D, as described by Yates in xe2x80x9cFundamentals of Fluidised-Bed Chemical Processesxe2x80x9d, Butterworths 1983, pages 14-15. This relationship can generally be ignored for small particles in large diameter beds.
Fischer-Tropsch processes which employ particulate fluidised beds in slurry bubble column reactors are described in, for example, U.S. Pat. Nos. 5,348,982; 5,157,054; 5,252,613; 5,866,621; 5,811,468; and 5,382,748. Slurry bubble column reactors operate by suspending catalytic particles in a liquid and feeding gas phase reactants into the bottom of the reactor through a gas distributor which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalytic system, they can be converted to both liquid and gaseous products. If gaseous products are formed, they enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered by passing the slurry through a filter which separates the liquid from the catalytic solids. In slurry bubble columns mixing is effected by the action of the rising gas bubbles.
In U.S. Pat. No. 5,776,988, a Fischer-Tropsch process is operated by passing liquid and gas through the reactor in an ascending flow so as to expand a particulate fluidised catalytic bed by at least 10% and up to 50% in relation to the height of the bed at rest and to place the catalyst in random movement in the liquid. By controlling the size and density of the catalytic particles, and the velocities of the gases and of the liquids, while taking into account the viscosity of the liquid and the operating conditions, the catalytic bed expands to a controlled height. The size of the catalyst is typically of mean equivalent diameter of between 100 and 5000 xcexcm. A commercial scale plant operated according to the process of U.S. Pat. No. 5,776,988 would require a catalytic bed of several meters in diameter and several meters deep.
However, a limitation to particulate fluidisation, particularly when applied to Fischer-Tropsch processes, is the marked relationship between the liquid fluidising velocity and the catalyst size and density. In order to avoid expansion of the bed out of the reaction vessel, a superficial liquid velocity must be employed below that of the terminal fall velocity, Ut, of the solid particles. To increase the xe2x80x9cwindowxe2x80x9d of operability of a commercial process, a larger catalyst particle size can be used which increases the terminal fall velocity, Ut, and hence allows a higher superficial liquid velocity to be employed. Generally, in order to prevent segregation of the catalyst particles, a very narrow particle size distribution is employed (often referred to as monosized catalyst particles).
A different type of fluidisation behaviour is observed when the solid particles have a density which is considerably higher than that of the fluidising liquid. This type of fluidisation is called xe2x80x9caggregativexe2x80x9d or xe2x80x9cbubblingxe2x80x9d fluidisation. The fundamental reasons for the transition from particulate to aggregative fluidisation is not well understood but, without wishing to be bound by any theory, an important factor is the density ratio, xcfx81s/xcfx81f, where xcfx81s is the density of the solid particles and xcfx81f is the density of the fluidising fluid. If the density ratio is high, aggregative behaviour is obtained; if the ratio is low, particulate fluidisation is observed. For particles having an average diameter in the range of 50 to 1000 xcexcm, fluidised with liquids having a density in the range of 700 to 1000 kg/m3, the transition from particulate to aggregative fluidisation occurs for solid particles having a density of greater than approximately 4000 kg/m3.
Expansion of an aggregative fluidised bed does not follow the relationship:
U=Utxcex5n. 
Instead, aggregative fluidised beds are characterised by the formation of particle free regions of fluidising liquid above the minimum fluidisation conditions. These particle free regions of fluidising liquid may be regarded as liquid xe2x80x9cbubblesxe2x80x9d or liquid xe2x80x9cvoidsxe2x80x9d.
Surprisingly, it has now been found that a Fischer-Tropsch process can be successfully operated using an aggregative fluidised catalytic bed.
The present invention relates to a process for the production of liquid hydrocarbon products by passing, at elevated temperature and pressure, synthesis gas and a fluidising liquid through a fluidised catalytic bed within a reaction zone, characterised in that the fluidised catalytic bed is an aggregative fluidised catalytic bed comprising a particulate Fischer-Tropsch catalyst having a density of greater than 4,000 kg/m3.